1. Field of the Invention
The present invention relates to a liquid crystal lens which can change a focal length in accordance with an electrical signal and a method of driving the liquid crystal lens. The present invention further relates to a lens unit including the liquid crystal lens, a camera module, and a capsule type medical device.
2. Description of the Related Art
A liquid crystal lens is expected to be applied to the autofocus function of a camera, an optical pickup device, and the like by advantageously utilizing the ability to electrically control the focus length of the lens. A conventional autofocus function has used a scheme of moving the position of the lens by using a voice coil motor. However, using a liquid crystal lens capable of changing its focal length eliminates a mechanical movable portion, leading to the downsizing and power saving of a lens unit.
Liquid crystal lenses are disclosed in, for example, patent literatures 1 and 2. The liquid crystal lenses disclosed in these literatures each are designed to use concentrically patterned transparent electrodes to form applied voltage gradients from the center of the lens to its periphery. This lens functions by changing the average tilt angle of liquid crystal molecules.
When using such a structure, it is necessary to use a lead wire pattern, which impairs rotational symmetry, to extract outside concentric electrodes on the central portion of the lens. This inevitably causes lens distortion. In addition, a fine transparent electrode pattern existing on a portion through which light from the lens passes has a refractive index that is too high to be ignored as compared with those of a glass substrate and liquid crystal. This causes scattering and reflection of light at a pattern end, which in turn interferes with imaging. According to the above literatures, it is necessary to finely control the voltages applied to many electrodes. This requires a complex driving circuit.
According to another example of the above literatures, a current is supplied to the patterned transparent electrodes to form voltage gradients applied to the liquid crystal through a voltage drop across a resistor. In this case, although voltage control is simple, the power consumption increases for driving operation.
Recently, as a compact camera module incorporated in a cellular phone or the like, a product added with a focus adjustment function and an angle-of-view adjustment function has been on the market. The focus adjustment function and the angle-of-view adjustment function are implemented by spatially moving some lenses in the lens unit using a voice coil motor and the like. This scheme, however, requires a mechanism for spatially moving lenses, and hence it is difficult to implement downsizing.
Medical practices conducted in medical fields use endoscopes capable of capturing stereoscopic images. An endoscope used in this case is, for example, a straight tube type endoscope using a fiber. Furthermore, a capsule type medical device incorporating a compact camera module has been on the market. The capsule type medical device can image the digestive organs of an object such as the small intestine and large intestine by being swallowed via the mouth. A capsule type medical device which captures a stereoscopic image of the digestive organ has also been researched and developed but has not been put into practice. One of the reasons for this is that it is very difficult to install two or more compact camera modules in a limited space.
Incorporating a fly-eye lens unit in a capsule type medical device can obtain a plurality of images having parallaxes necessary for stereoscopic image capturing. Using these images can generate a stereoscopic image of an observation region. Observing the stereoscopic image makes it possible to closely observe the surface state, e.g., irregularity, of a morbid region. This makes accurate examination and treatment possible.
Studies have also been made on a technique of transmitting driving power from outside the body to a capsule type medical device by a noncontact power transmission technique such as an electromagnetic induction scheme. At this time, there is a risk that strong electromagnetic waves may affect components in the capsule type medical device. In particular, the voice coil motor of a compact camera module incorporated in the capsule type medical device malfunctions due to the influence of the magnetic field generated by strong electromagnetic waves. As a consequence, the focus adjustment function and the angle-of-view adjustment function fail to accurately function.